A machine screw-threaded rod (straight, constant diameter, not tapered like a wood screw) and mating threaded nuts are widely used to fasten together or hold one or more objects (workpieces) to form a joint. At least one of the objects has a hole, a loop, a hook, or other means for positioning a sufficient portion of it around the rod and under an inside face of a nut, thereby clamping the object(s) when the nut is tightened. On the other side of the workpiece joint from the nut, the threaded rod has a radially extending portion that is fixed relative to the threaded rod. Typically this portion is either a bolt head or an object (e.g., a second object in the joint) to which the threaded rod is fixed to make a threaded stud extending outward from the object, preferably normal to a locally planar surface. The fixed element (head) serves the purpose of immobilizing any object that is advanced on the rod until it is stopped against the head and other intermediate objects, if any.
For the sake of convenience, the present disclosure may use terms interchangeably when referring to elements that are substantially functionally equivalent in the context being described. Such interchangeable equivalence should be readily apparent from the context of use, and furthermore is usually specifically stated herein. Therefore the use of one equivalent term should be considered as representing the same use of the other equivalent terms. For example, a threaded rod, rod (with threads), stud, bolt or bolt threads are used interchangeably. For example, a bolt head is considered equivalent to a radially extending portion of anything that is fixed relative to the threaded rod (e.g., an object's surface away from which an un-rotatable threaded stud extends).
Because there are so many uses for a simple removable fastener, it is often required to hold under various degrees of vibration and shock. Thus there is a great need for a threaded rod and nut type of fastener that will hold (lock) a joint together at a desired joint pressure (holding force, clamping force) even when subjected to vibration and/or shock loads. Vibration acts like a series of small shocks that, over time, can gradually “ratchet” the nut in an unscrewing direction to make the joint loosen (less joint pressure). Joint pressure is typically indirectly determined and specified in terms of an amount of torque that is required to tighten the nut enough to achieve the desired joint pressure.
For use with a single nut on a bolt or rod, there are a variety of well known “lock washers” in the prior art. One of the more effective versions is a Nord-Lock® Bolt Securing System washer set (NORD-LOCK Inc., 1051 Cambridge Drive, Elk Grove Village, Ill. 60007; www.nord-lock.com). In a typical example, this comprises two washers with interlocking ramps on one face of each, and a non-directional type of radial serrations on the other face of each. The two washers are placed between the nut and a work surface of the outermost object, with the ramped faces against each other. Then the nut is tightened enough to both achieve the desired clamping/joint force and to at least partially embed the serrations in both the working surface and the inside face of the nut. Given this, the interlocking ramps prevent relative rotation between the washers, and therefor between the work surface and the nut (because the serrations are frictionally binding adjacent surfaces to the respective washer). Thus torsional friction holds the nut in place on the threaded rod, and such friction is proportional to the axial load, i.e., the joint pressure.
Unfortunately the Nord-Lock type of lock washer is relatively expensive, as well as adding extra parts to a joint. Furthermore, they lose effectiveness when either the nut face or the working surface is too slippery due to, for example, a hardened surface that doesn't allow sufficient embedding of the serrations.
As is well known, there are many other designs for lock washers, none of which are very effective in holding a joint under vibration or shock.
A different approach to maintaining the joint pressure of nut on a threaded rod/bolt is to use “locknuts.” In its simplest form, a first (inner) nut is tightened on the bolt against the fastened work surfaces to a desired torque, which is indicative of an axial force (joint pressure, clamping force) that is desired for holding the fastened joint together. Then a second (outer) nut, designated as s “locknut” or “locking nut”, is tightened down against the first nut until a lock nut torsion spec is met, thereby increasing the friction (due to joint pressure) between the two adjacent nut faces to a level considered sufficient to keep the second nut from unscrewing relative to the first. Obviously the nut-nut joint pressure is accompanied by an equivalent axial reaction force that acts on the nut threads against the mating rod threads—in effect slightly stretching the threaded rod and/or distorting the mated threads within the first nut away from the same rod's threads that are mated with the threads inside the second nut.
There are now two effects acting to lock the nut(s) in place on the bolt, thereby maintaining a constant nut-to-work-surface joint pressure. Firstly, since vibration will generally be applied to the work pieces being held together by the nut and bolt, therefore the primary ratcheting effect will occur at the work surface-to-first nut interface, thereby attempting to unscrew the first nut. However, unscrewing the first nut acts in the direction of increasing the joint pressure between the two nuts, and increasing that pressure increases the friction between the nuts while also further distorting the bolt and nut threads. Secondly, although increased friction between nuts can make them act like a single nut effectively glued together, any unscrewing action must overcome resistance due to the thread distortions that reverse direction at the nut-nut interface. The result is that attempting to torque the two nuts as one is like trying to unscrew a nut over mashed or otherwise damaged threads.
Regardless of the theory, neither lock washers nor lock nuts are universally effective, particularly when vibration is a factor. For example, the vibration can cause the locked-together nuts to unscrew together for a small distance before returning to the screwed-on position; however, since the screwing/unscrewing actions are driven by the first nut which is against the work pieces, the nuts may slip against each other on the return, with the first nut being pulled away from the second nut. Even though this is a miniscule effect at first, long term vibration can repeat this step enough times to ratchet the nuts apart sufficient to reduce the nut-to-nut pressure to the point that the countervailing axial forces between nuts and thus the thread distortions are effectively eliminated. At that point the two nuts are no longer locked and can be rather easily unscrewed together, to unfasten the joint.
Another problem is that, to date, the more effective locking devices for fasteners tend to be more expensive and/or more difficult to use properly, often requiring special tools and time consuming extra work. Thus there is still an unmet, long-felt need for a more vibration resistant locking device for nut and bolt fasteners, preferably less expensive and simpler to use in a consistent and reliable way.
Additionally, there is a desire to have such a bolt locking device that can be removed at will, without having to destroy the fastener (e.g., cutting or breaking the bolt, e.g., stripping the threads of the nut(s) and/or bolt). After all, a nut and bolt fastener is often used instead of a rivet when the joint is intended to be un-fastenable at a later time, i.e., the fastener should be removable.
It is an object of the present work to provide a fastening method and device that satisfies the long-felt needs for a more vibration resistant, removable fastener.